The OLPC uses Open Firmware as the startup firmware. Instead of auto-booting into the Sugar (linux) operating system, you can ESC out into the firmware's environment and perform actions at its "OK" prompt.

At the "OK" prompt, Open Firmware understands the FORTH programming language, with certain built in commands.

More deductive souls might turn to http://wiki.laptop.org/go/Forth_Lessons and in lesson 5 find the command sifting. With an inkling that power might be involved, "ok sifting power" yields about a dozen commands. Near end is power-off. Bingo again!